I also run the Steeda springs. Shocks/stuts should be replaced, Koni adjustable or STR.T shocks are both nice. Ford GT500 strut mount (2010 Koni strut needs to be used with these) A lowered car will also need lower control arm relocation brackets (BMR), lower control arms should be upgraded (many good choices here), adjustable panhard bar. If driveshaft pinion angle is off you may also need adjustable upper control arm and good to have upgraded mount. I'm using stock 2 piece driveshaft so my pinion angle is good with the Steeda sport springs. This is pretty much the setup I'm using 1,000% better than stock setup.
I drive my car as a sports car, mostly rural roads so I can get pretty aggressive at times. With the stock setup the car felt so unsettled under braking I had to get all my braking done prior to turn. Now I can trail brake and not upset the car, much safer and pleasurable driving experience.
Now if your car is a city cruiser, I wouldn't bother with suspension first. Then I would do shifter first, the stock shifter setup is simply atrocious. Shifting is notchy, you will get locked out of third under hard acceleration. Y
You have to be very deliberate with shifting stock setup, unbelievable how much this slows down acceleration. I've tried half measures, such as better shift mounts, inserts, they help but not enough. I finally gave up and got the MGW Race Spec shifter, haven't installed but has highest marks. This shifter mounts directly to transmission, any shifter that uses a body mount will have shifting issues. The Blowfish Racing shifter mount is also highly rated, lower priced alternative to MGW. Royal Purple Syncromax tranny fluid and a stainless steel clutch line will finish off shifting issues hopefully for good (I will find out shortly after installing all these items).
I get my parts from LMR, CJ Pony Parts, Lethal Perf., American Muscle, Dallas Mustang Parts, Summit Racing, can't recall some other vendors.